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A Simplified Walk Through of the NB-27 Guide for Blowoff Vessels

*Note to Reader, Interchangeable Terms:  Blowoff Vessel, Blowoff Tank, Blowdown Tank

If you’re not an experienced engineer involved with boiler room equipment design, The National Board of Boiler and Pressure Vessel Inspectors’ guide for blowoff vessels can be a little daunting. Other than using the complex formulas themselves, part of the unease some individuals may experience is with reading the more open ended statements within the guide. There are so many factors that can effect a blowdown tank’s design it would be nearly impossible for the National Board to write comprehensive guide on each. So naturally they leave some things a bit ambiguous as to the way something can be designed or fabricated (Reference page 2 of NB-27 document).

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What are the Differences Between Intermittent Bottom Blowdown from a Boiler and Continuous Surface Water Blowdown from a Boiler?

Whether your boiler room has a fire tube type boiler or a water tube type boiler, flushing out dirty water and replacing it with fresh, clean water is a pivotal process in efficient boiling. With larger boilers, often times both top/surface water blowdown is used along with intermittent bottom blowdown. But what is the difference between these processes and why are each of them used? This article will go through each type of boiler blowdown and briefly discuss equipment used to handle each process.

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How long does it take for intermittent bottom boiler blowdown water to cool to a safe temperature inside of a blowdown tank?

Standard boiler blowoff tanks are designed to be large enough to handle one intermittent blowdown from a given boiler, send the flash steam that’s created upon entering the lower pressure vessel to atmosphere, and then allow the remaining condensate to cool by natural convection.
From there on, hot boiler blowdown water will immediately quench with the previously cooled condensate, rise above the overflow drain, and will then be sent to the city sewer at a safe, legal temperature.

This article is intended to explain in further detail how boiler blowoff tanks cool the boiler’s blowdown naturally, without any other instrumentation, electrical or otherwise. We will also discuss how long it takes to cool, what factors play a part in cooling, and some tips and recommendations when sizing a blow off tank for a boiler, or multiple boilers.

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How Madden offers fast lead times on boiler blowdown equipment: Get blowdown separators in as little as 1-2 weeks

Madden Manufacturing recently decided to invest time and resources into shortening lead times for our boiler bottom blowdown equipment product lines. The process of quoting, designing, and fabricating ASME certified pressure vessels for boiler blow off applications is meticulous. From the initial inquiry to date of shipment it is not uncommon for this process to take 4 months or more.

After weeks of going back and forth with the contractor to discuss the design and exactly what all is needed on the equipment, it can be frustrating to then wait 8-10 weeks for the fabrication stage of the unit(s).

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Where to buy the best boiler blowdown valve for continuous surface blowdown flow control?

There are several types of boiler blowdown valves and flow control devices available for controlling your boiler’s surface water blowdown. Madden Manufacturing makes an Orifice Meter type valve and they come in two designs, each offering 17 precise and consistent flow rates.

To operate at maximum efficiency, boilers need to continuously dispose of dirty boiler water. Dissolved solids and other sediments can cause your boiler to work harder than it needs to, to produce steam. Intermittently flushing bottom blowdown water and continuously blowing down a small percentage of the boiler’s surface water is a good way to maintain clean and efficient boiling.

And this is done with a meter or blowdown valve.

If you are wondering who has the best boiler blowdown valve for continuous top blowdown flow control, allow Madden to make a case for their Orifice Meter type blowdown valve.

Orifice Meter
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What’s the Difference Between a Blowdown Separator and a Blowdown Tank?

Overview

Blowdown Tanks and Blowdown Separators, sometimes called blowoff tanks and separators, are two different types of boiler blowdown equipment that serve the same purpose: receiving intermittent, bottom boiler blowdown and to then cool the blowdown to a safe temperature for disposal.

This blog is written to explain the differences in their function.

Tank
Tank
Separator
Separator
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